1. Field of the Invention
The present invention relates to an analog input device mounted in, for example, a controller for a game machine, and more particularly, to an analog input device that improves the output characteristic with respect to changes in load or stroke during a key-pressing operation.
2. Description of the Related Art
FIG. 7 is a cross-sectional view showing a state of a known analog input device before deformation, and FIGS. 8A and 8B are graphs showing the input-output characteristic of the analog input device. In FIG. 8A, the input represents the load, and the output represents the number of bits obtained by subjecting the analog output voltage between a movable electrode and a resistor to A/D conversion. In FIG. 8B, the input represents the stroke, and the output represents the number of bits, in a manner similar to that in FIG. 8A.
In an analog input device 1 shown in FIG. 7, an analog-quantity generating unit 3 is provided on the inner bottom face of a base 2 to output analog signals. An elastically deformable rubber member 4 is provided on the analog-quantity generating unit 3. A key top 5 is supported at the top of the rubber member 4 so that it can move in the Z1- and Z2-directions.
The rubber member 4 has an annular base portion 4a on its outer periphery, and a thin-walled support portion 4b extending from the base portion 4a toward the axis O-O. A pressing portion 4c is provided integrally with the leading end of the support portion 4b, and protrudes in the Z2-direction.
In the analog-quantity generating unit 3, a resistor 3b provided on the bottom face of the base 2 opposes a movable electrode 3a with a small gap therebetween in the Z1-direction.
In this analog input device 1, when the key top 5 is pressed in the Z2-direction, the entire rubber member 4 is elastically deformed. Since the pressing portion 4c depresses the movable electrode 3a of the analog-quantity generating unit 3, the movable electrode 3a is bent into contact with the surface of the resistor 3b. 
In this case, since the contact area between the movable electrode 3a and the resistor 3b varies depending on the load or stroke of the key top 5 during a key-pressing operation, the resistance therebetween correspondingly varies. Therefore, when the key top 5 is depressed in a state in which a fixed resistor is connected to the resistor 3b in series and a predetermined voltage is applied between the movable electrode 3a and the fixed resistor, a voltage (analog signal output) output from both ends of the fixed resistor in accordance with the load applied to the key top 5 or the stroke of the key top 5 is detected.
The above conventional art is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2002-093274.
In the above-described analog input device 1, however, the load F applied to the key top 5 is transmitted to the rubber member 4 through an annular pressed portion 4d at the upper end, and is concentratively applied from the rubber member 4 to the movable electrode 3a through one point B at the leading end of the pressing portion 4c. 
When it is assumed that a load F is applied to a point A on the pressed portion 4d of the rubber member 4, a moment M centered on the point B is applied to the point A in the direction shown by the arrows. The moment M is proportional to the distance between the points A and B and a component F·sin θ of the load F, and distorts the rubber member 4. Since the rubber member 4 is distorted before the movable electrode 3a touches the resistor 3b, a resistance is not produced between the movable electrode 3a and the resistor 3b until the rubber member 4 is distorted to some extent and the movable electrode 3a is pressed into contact with the resistor 3b. 
This means that a dead zone W appears in the conventional analog input device. In the dead zone W, an output is not presented until the load exceeds 0.39 N (approximately equal to 40 gf) (see FIG. 8A) or the stroke exceeds 0.7 mm (see FIG. 8B) even when the key top 5 is pressed.
In this way, in the above analog input device 1, the load or stroke of the key top 5 is distributed by the rubber member 4, and the force for pressing the movable electrode 3a is not efficiently transmitted. For this reason, the output response to the input is not quick, and the output is produced only by applying at least a predetermined load or stroke. Consequently, the linearity of the input-output characteristic is poor.